Field of the Invention
The present invention relates to a system and method for controlling a temperature of a fuel cell stack, and more particularly, to a method for controlling a temperature of a fuel cell stack by adjusting a voltage of a cooling pump and a power converter.
Description of Related Art
A fuel cell vehicle includes a fuel cell stack in which a plurality of fuel cells used as a power source are stacked, a fuel supplying system that supplies hydrogen, or the like, which is fuel, to the fuel cell stack, an air supplying system that supplies oxygen, which is an oxidizing agent required for an electrochemical reaction, a water and heat management system that adjusts a temperature of the fuel cell stack, and the like. The fuel supplying system reduces a pressure of compressed hydrogen in a hydrogen tank and supplies the compressed hydrogen to an anode of the fuel cell stack, and the air supplying system supplies external air suctioned by operating an air blower to a cathode of the fuel cell stack.
When hydrogen is supplied to the anode of the fuel cell stack and oxygen is supplied to the cathode of the fuel cell stack, hydrogen ions are separated by a catalytic reaction in the anode. The separated hydrogen ions are transferred to an oxidizing electrode, which is the cathode, through an electrolyte membrane, and the hydrogen ions separated in the anode generates an electrochemical reaction together with electrons and the oxygen in the oxidizing electrode, such that electric energy may be obtained. In particular, electrochemical oxidization of the hydrogen is generated in the anode, and electrochemical reduction of the oxygen is generated in the cathode. Further, electricity and heat are generated due to movement of electrons generated by the above-mentioned process, and water vapor or water is generated by a chemical bonding of the hydrogen with the oxygen.
Additionally, to discharge hydrogen, oxygen, and the like that do not react to byproducts such as water vapor, water, and heat generated in an electric energy generating process of the fuel cell stack, a discharge apparatus is provided, and gases such as water vapor, hydrogen, and oxygen are discharged into the atmosphere through a discharge path. Components such as an air blower, a hydrogen recirculation blower, and a water pump for operating the fuel cell are connected to a main bus terminal to facilitate a start-up of the fuel cell, in which the main bus terminal may be connected to various types of relays which facilitate cutting off and connection of power and a diode which prevents a reverse current from flowing in the fuel cell.
A fuel cell system used in a hydrogen fuel cell vehicle which is one of the environmentally-friendly vehicles is configured to include a fuel cell stack that generates electric energy from an electrochemical reaction of reaction gas, a hydrogen supplying apparatus that supplies hydrogen, which is fuel, to the fuel cell stack, an air supplying apparatus that supplies air including oxygen, which is an oxidizing agent required for an electrochemical reaction, to the fuel cell stack, a heat and water management system that optimally adjusts an operation temperature of the fuel cell stack by discharging heat, which is electrochemical reaction byproducts of the fuel cell stack, to the exterior and performs a water management function, and a fuel cell system controller that operates the fuel cell system.
By the configuration, the fuel cell stack generates electric energy from the electrochemical reaction of hydrogen and oxygen which are reaction gas and discharges heat and water which are the reaction byproducts. Therefore, the fuel cell system essentially includes an apparatus for cooling a fuel cell stack to prevent the temperature of the fuel cell stack from increasing. In particular, a polymer electrolyte membrane fuel cell (PEMFC) has merits of a rapid start-up time and a rapid power conversion reaction time due to a low operation temperature while having a high power density; however, the PEMFC requires water and therefore needs to be operated at a temperature of about 100° C. or less.
Generally, a cooling system for maintaining the fuel cell stack at an optimal temperature in the fuel cell system for a vehicle has widely adopted a water cooling type which cools the fuel cell stack by circulating water through a cooling water channel in the fuel cell stack.
A temperature control system of the fuel cell system is illustrated in FIG. 1. As illustrated in FIG. 1, the temperature control system of the fuel cell system includes a radiator 60 and a cooling fan 21 that discharges heat from cooling water to the exterior, a cooling water line 31 disposed between a fuel cell stack 10 and the radiator 60 to circulate the cooling water, a bypass line 32 and a 3-way valve 33 for bypassing the cooling water to prevent the cooling water from passing through the radiator 60, and a cooling water pump 70 for pumping the cooling water and delivering the pumped cooling water through the cooling water line 31. Particularly, the bypass line 32 is a cooling water line which does not pass through the radiator 60 by being branched from the cooling water line at upstream and downstream sides of the radiator to bypass cooling water and the 3-way valve 40 serves to selectively adjust a flow of cooling water between a main line and the bypass line 32 which does not pass through the radiator.
Meanwhile, the fuel cell stack has both of a flooding phenomenon wherein water floods and a dry-out phenomenon wherein water is supplied in insufficient amounts. To improve the phenomena, the temperature of the fuel cell stack needs to be maintained within a normal range (e.g., in which the water does not overflow or dry out). In other words, a need exists for a method for preventing the flooding phenomenon and the dry-out phenomenon by adjusting the temperature of the fuel cell stack while minimizing power consumption by optimizing the operation of the cooling water pump and the cooling fan.